In grinding surfaces upon workpieces, the always difficult problem of maintaining a proscribed geometry to a low tolerance is compounded substantially whenever the grindability characteristics are variable upon a single surface. Due to the variable grinding characteristics, different metal removal rates may result for different sections upon the single surface whenever the grinding wheel-workpiece interface force is constant. For example, for a constant engagement force between the grinding wheel and a workpiece, the metal removal rate for an extremely hard material will frequently be less than the corresponding removal rate for a softer material. If the extremely hard material is intermittently inserted into a section of the surface of a parent workpiece of a softer material, as is done in a variety of applications, the resulting surface to be ground has disparate grindability characteristics. The disparity in the grinding characteristics of the two materials obviously presents problems in grinding surfaces which require a high degree of geometric exactitude.
Different grindability characteristics are also presented by a surface which is partially interrupted, as for example, interruptions, by holes and/or flats. Holes and/or flats vary the contact area between the grinding wheel and the workpiece and frequently vary the unit force applied to the contacted portions of the surface. Since the metal removal rate is most commonly a function of the applied unit force, it varys accordingly. Consequently, holes and/or flats frequently have presented problems in forming proscribed geometrics during the grinding process in prior art machines.
Further, virtually all grinding machines experience some deflection in the grinding wheel support system whenever the wheel is urged against the workpiece. This deflection results in a reactant spring force which is applied against the workpiece, the magnitude of the spring force being proportional to the product of the support system spring rate and the magnitude of the deflection. Deflection is particularly pronounced when the grinding wheel is mounted upon a quill in cantilevered support, as the wheel support system not only experiences deflection in the bearings and the wheel itself, but the quill supporting the grinding wheel inherently defects substantially whenever a force, such as that resulting from engagement with a workpiece, is applied to its end portion. The unequal resistances offered by disparate grinding characteristics upon different sections of a single grinding surface result in different deflections and consequently different metal removal rates for the different sections of the surface; and this nonuniformity of metal removal rates produces a part which does not meet the proscribed geometrical tolerances.